This invention relates to hydrocarbon refining and, in particular, to the production of highly aromatic hydrocarbon products.
The processing of gas oils has a long history in the petroleum refining industry. Recognizing the need to maximize useful and value added products derived from petroleum feedstocks, modern refineries chemically treat, distill, catalytically crack, thermally treat, extract, and otherwise process crude oil blends to produce a wide variety of products. Enhanced production of light ends and gasoline blending stocks yields increased volumes of heavy and residual oils that have significantly lower values than other refinery streams. Indeed, the production of many gas oils is frequently the limiting factor in overall refinery production rates.
Gas oils, as the term is used in this application; are hydrocarbon streams with an ASTM D-2887 SimDist initial boiling point (IBP) greater than 195.degree. C., and a 5% temperature greater than 200.degree. C. Gas oils are typically processed in one of two ways. Such oils may be thermally or catalytically "cracked" to produce lighter, i.e., lower molecular weight, hydrocarbon fractions with increased hydrogen/carbon ratios for eventual use as motor gasoline or distillate blending stocks. Alternately, gas oils are subjected to extreme heat for extended periods of time to produce solid petroleum coke and lighter hydrocarbon streams. Depending on local production constraints and product demand, one or both process may be utilized.
Applicants have discovered a third alternative process applicable to such gas oils. Recognizing that gas oils, such as FCC cycle oils, FCC gas oils, FCC decant oils, aromatic vacuum gas oils, atmospheric tower gas oils, and the like consists of a core structure of two or more aromatic rings with substantially aliphatic chains attached thereto, Applicants have discovered a novel process for breaking the aliphatic chains from the aromatic core. The process yields a mixture of substantially aliphatic lighter hydrocarbons, heavy aromatic oils, and very heavy residual oils, which can be readily separated by standard refining practices. The lighter hydrocarbons may be used as blending stocks for gasoline, distillates, or other re-refining, the heavy aromatic oils used as high value added polymer plasticizers, and the very heavy residual oils used for the production of asphalt, coke, pitch, and the like.
In typical cracking processes, the gas oil feedstock is either thermally or catalytically cracked. That is, the gas oil is broken into smaller molecules, and conditions adjusted to minimize the production of higher molecular weight materials. Catalytic cracker operation is primarily gas phase, and the time the feedstock remains in the reactor is relatively short, on the order of seconds.
Alternately, the gas oil may be coked. Unlike the cracking process described above, coking processes attempt to remove light ends from heavier materials as quickly as possible to minimize the percentage of feed that is converted to petroleum coke. In coke production, the gas oil is quickly heated to a temperature in excess of 425.degree. C., light ends are flashed off in a coking drum, and the heavy materials maintained at an elevated temperature for a period of hours. The time at temperature causes the heavy oil to polymerize into a solid.
Unique to the current invention is the use of relatively severe conditions for relatively brief periods of time to partially crack the gas oil. The current invention severs the bonds connecting the aliphatic chains from the more rigidly structured, highly coordinated aromatic complexes in the oil. Additionally, a portion of the aliphatic chains may be cracked to form shorter, lower molecular weight hydrocarbons, while still not appreciably cracking the aromatic structure.